1. Field of the Invention
The present invention relates to a signal transmission bus for transmitting signals between a plurality of circuit boards and a signal processing apparatus utilizing that signal transmission bus.
2. Description of the Related Art
New developments in the art of very-large-scale integration (VLSI) have significantly enhanced the functions of daughter boards used by data processing systems. The growing capabilities of circuits requires connecting an increasing number of signals to each of configured circuit boards. The trend has entailed the adoption of a parallel architecture wherein the data bus board (i.e., mother board) for interconnecting circuit boards through a parallel bus structure comprises a large number of connectors and connecting lines. The operating speed of the parallel bus structure is boosted conventionally by installing connecting lines in a plurality of layers and by miniaturizing the wiring layer setup. In such an architecture, the line capacity of connection wiring and wiring resistance produce signal delays that determine the operating speed of the parallel bus structure. In turn, the operating speed of the parallel bus structure may constrain the processing speed of the system. Another major constraint on the efforts to increase the processing speed of the system is the problem of electromagnetic interference (EMI) stemming from the large-scale integration of the parallel bus connection wiring.
In an attempt to solve the above problems and to enhance the operating speed of the parallel bus structure, researchers have examined the use of an intra-system optical connection technique called the optical interconnection. Various applications of the optical interconnection technique varying with the system configuration have been disclosed. Some of these application are discussed illustratively by Sadaji Uchida in the paper presented to the Lecture Meeting on Circuit Implementation, 15C01, pp. 201-202, and by H. Tomimuro et al., in "Packaging Technology for Optical Interconnects," IEEE Tokyo Section, Denshi Tokyo No. 33, pp. 81-86, 1994.
One of the simplest applications is depicted illustratively by Christopher et al., in "Optical Interconnection Foundations and Applications," Artech House Inc., Boston, London, 1994. Chapter 6 of this publication describes a typical setup wherein circuit boards are interconnected by a single optical fiber cable and the interface between the boards is composed of light-emitting and light-receiving elements as well as parallel-serial conversion circuits. In this setup, the electronic circuits on the circuit boards are connected by a 32-bit parallel electrical bus structure. With the clock time assumed to be about 50 MHz, the parallel-serial conversion circuits between the parallel electric bus and the serial optical bus are required to operate approximately at 2.7 GHz.
Where parallel electrical signals are converted to serial optical signals in order to interconnect the circuit boards by the optical fiber cable, as in the above example, the data transmission rate of the bus is determined by the operating speed of the parallel-serial conversion circuits. If the bit count of the electronic circuitry is increased to 64 or to 128, it is difficult to enhance the data transmission rate correspondingly. To improve the operating speed of the parallel-serial conversion circuits for higher data transmission rates requires installing expensive electronic circuits. Furthermore, boosting the operating speed requires the parallel-serial conversion circuits to consume drastically large amounts of power.
Japanese Unexamined Patent Application No. Hei 2-41042 discloses an example in which an optical transmission system using high-speed, high-sensitivity light-emitting/receiving devices is applied to the data bus. The proposed application involves a parallel optical data bus structure for loop transmission between contiguous circuit boards incorporated in a system frame, each circuit board carrying light-emitting and light-receiving devices on both sides, the devices being optically connected across the space.
In the system cited above, signal light sent from one circuit board is converted by the adjacent circuit board first from optical to electrical format and then from electrical to optical format before the signal light is forwarded to the next adjacent circuit board. That is, the optical-to-electrical and electrical-to-optical conversions are repeated by each of the circuit boards included in the system frame, until the signal is transmitted through all the circuit boards configured. This means that the signal transmission speed depends on and is restricted by the optical-to-electrical and electrical-to-optical conversion speeds of the light-receiving and light-transmitting devices mounted on all circuit boards. Because data is transmitted from one circuit board to another via optical connections across the free space between the light-receiving and light-emitting devices, it is required that the light-receiving and light-emitting devices mounted on both sides of each of the contiguous circuit boards be optically aligned and that all circuit boards be optically connected.
Because the circuit boards are optically connected across the free space, faulty transmission of data therebetween may result from cross talk between contiguous optical data transmission routes. Defective data transmission may also occur when the signal light is diffused by ambient factors such as dust inside the system frame. Furthermore, since the circuit boards are serially connected, dismounting any one of the boards disrupts the integral flow of data; an extra circuit board must fill the vacated board position. In other words, the number of the circuit boards configured is fixed, and none of the boards can be freely detached.
Another application of the optical interconnection is disclosed in Japanese Unexamined Patent Application No. Sho 61-196210 regarding techniques for optical data transmission between circuit boards across the free space. What is disclosed is a system of optically connecting circuit boards via optical transmission routes across the free space, the system involving plates each having two parallel surfaces and located opposite to a light source, the plate surfaces bearing diffraction grating and reflector elements. With this system, a light beam generated from a single point is transmitted to another point that is fixed. This system is incapable of interconnecting all circuit boards in the same manner as the electrical bus structure. The use of the free space necessitates complicated optics with its optical elements difficult to align. Misalignment of optical elements can produce cross talk between contiguous optical data transmission routes, resulting in faulty data transmission. The information about the connection between the circuit boards is determined by the diffraction grating and reflector elements mounted on the plate surfaces. This makes it impossible to detach or attach circuit boards as needed, which reduces the degree of flexibility where the system configuration is desired to be altered.